Exercise as an Adjuvant to Bone and Cartilage Regeneration Therapy

: This article provides a brief review of the ontogeny of chondrocytes and the pathophysiology of osteoarthritis (OA), and details how physical exercise improves the health of osteoarthritic joints and enhances the potential of mesenchymal stem cells for successful transplantation therapy. In response to exercise chondrocytes increase their production of glycosaminoglycans, bone morphogenic proteins and antiinflammatory cytokines and decrease their production of proinflammatory cytokines and matrix degrading metalloproteinases. These changes are associated with improvements in cartilage organization and reductions in cartilage degeneration. Studies in humans indicate that exercise increases peripheral blood recruitment of bone marrow-derived mesenchymal stem cells (BM-MSC) and upregulates BM-MSC expression of osteogenic and chondrogenic genes, osteogenic micro-RNAs, and osteogenic growth factors. Rodent experiments are uniform in demonstrating that exercise enhances the osteogenic potential of BM-MSC while diminishing their adipogenic potential, and that exercise done after stem cell implantation may benefit stem cell transplant viability. Physical exercise also exerts a beneficial effect on the skeletal system by decreasing immune cell production of osteoclastogenic cytokines interleukin (IL)-1β, tumor necrosis factor (TNF)-α, and interferon (INF)-γ while increasing their production anti-osteoclastogenic cytokines IL-10 and transforming growth factor (TGF)-β. In conclusion, physical exercise done both by stem cell donors and recipients may improve the outcome of mesenchymal stem cell transplantation. topics included exercise and osteoarthritis, exercise and osteoporosis, exercise and bone remodeling, exercise and bone homeostasis, exercise and mesenchymal stem cells, exercise and mesenchymal stromal cells, exercise and hematopoietic stem cells, exercise and osteoblasts, exercise and osteocytes, exercise and osteoclasts, exercise and osteogenic hormones, exercise and pro-inflammatory cytokines, exercise and anti-inflammatory cytokines, exercise and myokines, stem cell therapy of osteoarthritis, and stem cell therapy of osteoporosis. Keywords included osteoarthritis, osteoporosis, osteoimmunology, osteocytes, osteoblasts, osteoclasts, mesenchymal stem cells, hematopoietic stem cells, cytokines, interleukins, macrophage colony stimulating factor, receptor activator of NFkB ligand, receptor activator of NFkB, osteoprotegerin, tumor necrosis factor-α, and sclerostin. and analyzed for alkaline phosphatase, collagen synthesis and formation of mineralized nodules. They found that exercise increased BM-MSC osteogenesis


Introduction
In the global burden of disease 2010 study, osteoarthritis accounted for 17,135 years of life lived with disability (YLD), an increase of 64% when compared to YLD of 1990. Overall, musculoskeletal disorders (which included inflammatory causes of arthritis) accounted for 6.8% of total YLDs [1] with osteoarthritis ranked as the 11 th leading cause of disability worldwide [2].
In addition, an estimated nine million osteoporotic fractures occurred globally in the year 2000 resulting in a loss of 5.8 million Disability Adjusted Life Years and accounting for 0.83% of the global burden of noncommunicable disease [3]. In 2011, 1.7 million people were hospitalized in the United States for osteoporosis-related fragility fractures at a cost of over 70 billion dollars [4].
In 2015, the prevalence of osteoporotic vertebral fractures in caucasian women ≥ 50 years of age from North America was estimated at 20-24%; in Europe, the prevalence varied between 18% and 26% [5]. Overall, osteoporosis is estimated to occur in one in three women and one in 12 men [6].
And the global prevalence of osteoarthritis and osteoporosis is expected to increase as the average age and weight of the World's population increases.
Physical exercise has long been recognized as an essential factor in the maintenance of bone health, particularly during adolescence when ~ 50% of bone mass accretion occurs [4]. The 2019 American College of Rheumatology/Arthritis Foundation guidelines for the management of osteoarthritis of the hip and knee emphasized the importance of regularly performed physical exercise [7]. Both traditional (resistance, aerobic and flexibility) and non-traditional (Tai Chi, yoga, aquatic) exercises have been shown to be effective in the management of knee and hip osteoarthritis [8]. In addition, clinical trials have shown that multicomponent training (aerobic and resistance exercises) is effective in increasing bone mass in osteoporotic women [9]. In this regard The World Health Organization recommends that adult men and women should accumulate at 4 least 150 min of moderate intensity physical activity per week and young people aged 5-17 years should accumulate at least 60 min of physical activity of moderate to vigorous intensity daily [10].
There is increasing interest in treating bone and cartilage disease with mesenchymal stem cell (MSC) implants [11][12][13][14][15][16][17]. Bone marrow-derived MSC, adipose tissue derived MSC, and umbilical cord-derived MSC been shown to have beneficial effects in osteoporotic animals, and intravenous administration of autologous fucosylated bone-marrow-derived MSC to patients with osteoporosis is currently in a phase I clinical trial. MSC transplants have the potential to repair bone by differentiating into osteoblasts and chondrocytes and by secreting growth factors that stimulate osteoblastogenesis and angiogenesis and inhibit osteoclastogenesis [18].
In this article I review the pathophysiology of osteoarthritis (OA) and detail how physical exercise improves the health of articular cartilage and chondrocytes in OA and enhances the potential of mesenchymal stem cells for successful transplantation therapy. I also discuss how exercise protects the skeletal system by upregulating the production of antiosteoclastogenic cytokines and downregulating the production of osteoclastogenic cytokines by chondrocytes and peripheral blood mononuclear cells

Materials and Methods
This narrative review is on exercise as an adjuvant to bone and cartilage regeneration therapy. The research strategy included the following: 1. defining the key topics; 2. identifying key words or synonyms that represent each of the key topics; 3. an online PubMed search of key topics and key words; and 4. a refinement of the search based on initial findings. Data restrictions included articles with identical samples and identical outcomes, identical samples with different outcomes, increased samples and identical outcomes, and decreased samples with identical outcomes. Key 6 chondrocytes become senescent, a phenotype associated with the secretion of proinflammatory cytokines and matrix degrading proteases and a reduction in the secretion of antiinflammatory cytokines [21][22][23]. Senescent chondrocytes eventually undergo apoptosis terminating articular cartilage synthesis [21]. Other metabolic derangements involving transforming growth factor (TGF)-β, fibroblast growth factor (FGF)-2, FGF-18, growth differentiation factor (GDF)-5, and hypoxia-induced factor (HIF)-2a may also contribute to the pathogenesis of osteoarthritis [19].
Studies on menisci, which contain multiple subpopulations of cells responsible for tissue repair and maintenance ("fibrochondrocytes") have shown that their production of interleukin (IL)-1 is elevated in OA (109-288 pg/mL). IL-1 is a potent proinflammatory and osteoclastogenic cytokine whose effects on menisci and articular cartilage are catabolic [24].

Exercise and osteoarthritis
Although regularly performed moderate intensity exercise is recognized as the mainstay treatment of OA [7,8] there are a limited number of studies sampling constituents of the OA joint before and after supervised exercise training of men and women. One of these was published by Roos and Dahlberg and involved 45 subjects who had undergone medial meniscus resection 3-5 years prior to the study and were at risk of developing OA. Subjects underwent supervised exercise training 3 times weekly for 4 months or were assigned to a noninterventional group. All subjects had the content of their knee cartilage glycosaminoglycan content assessed by delayed gadoliniumenhanced magnetic resonance imaging. Exercise increased cartilage levels of glycosaminoglycan in proportion to the level of physical activity [27].
In a similar study, Munukka and associates assessed the effects of 12 months of leisure time physical activity on the glycosaminoglycan content of femoral cartilages in 76 post-menopausal women with knee OA using delayed gadolinium-enhanced magnetic resonance imaging. They also found that exercise increased the amount of cartilage glycosaminoglycan [28].
Iijima and associates studied the effects of 2-4 weeks of treadmill walking in 24 male Wistar rats with induced damage to their knee joints using micro-computed tomography, histology and immunohistochemistry analysis. They found that exercise prevented the progression of posttraumatic bone and cartilage lesions and increased BMP-2 and BMP-6 expression in the joint superficial zone chondrocytes [29]. 9 Assis and associates studied the effects of aerobic exercise training on an experimental model of knee osteoarthritis in 50 male Wistar rats. Twenty of the rats were trained on treadmills 3 days/week at 16 meters/minute for 50 minutes/day for 8 weeks. The exercising and control rats were sacrificed, and their knee joints assessed by histologic, morphometric and immunohistochemical analysis. Compared to the controls, exercising animals had a better pattern of cartilage organization and less cartilage degeneration. Exercising animals also had lower chondrocyte nuclear or nucleolar expression of IL-1β, caspase-3 and matrix metalloproteinase (MMP)-13, confirming the ability of aerobic exercise to downregulate proinflammatory and proteolytic pathways in this model of OA [30].

Exercise and mesenchymal stem cells
The author is using the International Society for Cell and Gene Therapy committee's recommendation that the acronym "MSC" be used for both mesenchymal stem cells and mesenchymal stromal cells and that the MSC acronym be preceded by "BM" for bone marrow origin and "AD" for adipose tissue origin [31].

Studies in rodents
Using groups, but the effect was greater in young animals than in adults [37]. Using mice, Wallace and associates measured the effects of 5 days of treadmill exercise (30 minutes/day) on BM-MSC and found that exercise increased their osteogenic potential [38]. Yamaguchi measure the effects of exercise on the ability of BM-MSC obtained from male Wistar rats to repair experimentally induced femoral groove osteochondral defects in female Wistar rats. Two weeks after BM-MSC were injected into the defective joints, rats were either sedentary or subjected to 2, 4, or 8 weeks of treadmill exercises performed 5 days/week at 12 meters/minute for 30 minutes; the animals were then sacrificed, and their joints subjected to immuno-histochemical staining. Compared to the sedentary group, they found that exercise enhanced cartilage repair and concluded that their study "highlights the importance of exercise following cell transplantation therapy" [39].

Studies in humans
There  [44]. Sumanasinghe and associates seeded human BM-MSC in 3D type I collagen matrices and subjected them to 0%, !0%, or 12% uniaxial cyclic tensile strain at 1 Hz for 4 hours/day for 7 or 14 days. They found that BMP-2 mRNA expression and BMP-2 production was upregulated in the strain samples as compared to controls indication that mechanical strain of the type associated with exercise can induce osteogenic differentiation of human BM-MSC [45].
In summary, experiments in humans, while limited in number, have shown that exercise upregulates MSC and HSC recruitment, enhances osteogenic, chondrogenic and apoptotic gene expression, and upregulates the expression of osteogenic miRNAs and the secretion of growth factors ( Figure 3, Table 1).

Exercise and osteoclastogenic and antiosteoclastogenic cytokines.
In a before and after trial involving 43 healthy adults Smith and associates measured the effect of six months of combined aerobic, resistance, and flexibility exercises on the production of osteoclastogenic cytokines (IL-1α, TNF-α), anti-osteoclastogenic cytokines (TGF-β, IL-4, IL-10), and cytokines with variable effects on osteoclastogenesis (interferon (IFN)-γ, IL-6) by cultured mitogen-stimulated peripheral blood mononuclear cells (PBMC). Also measured were serum markers of bone formation (osteocalcin) and bone resorption (C-terminal telopeptides of Type I collagen). Exercises done on an average of 2.5 hours a week attenuated the production of osteoclastogenic cytokines and enhanced the production of antiosteoclastogenic cytokines ( Figure   2). These changes were accompanied by a 16% reduction in collagen degradation products and a 9.8% increase in osteocalcin levels. They concluded that "Long-term moderate intensity exercise exerts a favorable effect on bone resorption by changing the balance between blood mononuclear cells producing osteoclastogenic cytokines and those producing antiosteoclastogenic cytokines" [46]. In summary, exercise upregulates PBMC production and serum levels of antiosteoclastogenic cytokines and downregulates PBMC production and serum levels of osteoclastogenic cytokines.
In one study, exercise increased intraarticular and perisynovial levels of IL-10 in patients with knee OA (Figure 4).  [53]. Preclinical trials have shown that ACT is successful in producing hyalinelike cartilage regrowth [54][55][56] with reasonable long-term durability [57]. However, this method is associated with up to 40% dedifferentiation of chondrocytes during culturing and/or after the transplant [58]. Whether exercise will improve the outcome of ACT is yet to be determined.
Whether other joint preservation techniques are benefitted by exercise is also unknown.
Osteoplasty involves drilling or punching of holes through the subchondral plate at the site of the chondral defect; this incites an inflammatory response which includes the mobilization of BM-MSC to the articular surface [59]. Since exercise has been shown to increase circulating levels of BM-MSC, exercise done before and after osteoplasty has the potential to improve the results of this regenerative technique.
The evidence provided in this review supports a policy of recommending physical exercise for both BM-MSC donors and recipients. In addition, although not yet documented, exercise may benefit ACT donor/recipients and persons undergoing osteoplasty.
Recommending exercise for patients undergoing regenerative procedures has the added benefit of reducing their risks for ischemic cardiovascular disease, hypertension, diabetes mellitus, the metabolic syndrome, and certain forms of cancer [60].
The recommendation of the World Health Organization that adults undergo a minimum of 50 minutes of moderate intensity exercise three times weekly [10] should be sufficient to provide the aforementioned benefits.

Conclusion
Physical exercise done by both stem cell donors and recipients may improve the outcome of mesenchymal stem cell transplantation.
Funding: This research received no external funding